I have to think about the security of my business. The way to get me to stop running my own SMTP and IMAP is to provide a service which is: - auditable (our stored messages by us; the whole system by a third party) - reliable (99.9% uptime, that's not so awful) - resilient (if you lose a server or a data center, I shouldn't have to worry about it. Ideally I don't notice anything except perhaps a slowdown) - guaranteed (with a hefty bond against security issues and another one against prolonged downtime, not a pro-rated refund)

Those are the gotta-haves. If you provide a secure clearinghouse to talk to our clients and vendors, that would be a major selling point. It has the chicken-and-egg problem, of course, but also the network value effect.